As the demand for electricity in today's society continues to grow, it is desirable to produce power as efficiently as possible. Use of steam to produce power through turbines is being increasingly expanded, both by greater numbers of turbines and by longer hours of operation. These increasing demands make it necessary for such turbines to be used in the most cost and energy efficient manners possible.
Older turbines are often refurbished with newer components to improve efficiency. Such components can include items such as nozzle blocks and reaction blading and will, because of improved manufacturing techniques and use of harder materials, often result in obtaining closer tolerances. This is also true in newer turbines. Thus, there is less flow area and since the harder materials do not erode as rapidly as did old steam path materials, keeping the flow path itself clean becomes essential in order to maintain efficient operation. The increasing size of the flow path area due to erosion that was characteristic of the old materials would in some cases, compensate for the deposit of buildup material and for a while allow an adequate steam flow passage to be maintained. This is not always the case with newer designs.
Accordingly, the closer tolerances and harder materials in conjunction with the improved operating performance resulted in conditions more sensitive to deposit buildup and require more frequent and better cleaning.
When materials do build up inside the turbine, it is important remove them as quickly as possible. One approach often used is to tear the high pressure (HP) turbine apart and blast the deposits off the internal parts with a grit or sand medium. This method involves high cost and a long period of down time during which the turbine cannot be used. In 1984, the cost of operating a HP turbine with efficiency and load curtailment was estimated to be $1.036 million annually and the cost of grit blasting was estimated at $350 thousand.
Another more cost effective method for removing deposits is to chemically clean the turbine and its internal parts. This method has been successfully performed by utility companies to combat load losses caused by chemical deposits in the steam paths of turbines.
However, to perform a chemical cleaning of a HP turbine, chemical cleaning agents, such as cleaning foams, must be injected into the main steam system of the turbine and must follow the same path followed by the steam during normal operation. Injection points would have to be made and located in the main steam loops that feed the steam to the turbine's governing valve system. To incorporate these injection points, it was necessary to penetrate the main steam lines followed by certain machining steps in order to install a connection. This method had an initial estimated installation cost of $50,000. Thereafter the connection would require welding, x-ray testing, and stress-relieving measures prior to using the connection. Also, in some instances the structural integrity of the pressure vessel may have been altered and that would have to be repaired.
After cleaning had been completed, a cap would then have to be installed covering the connection and this required the additional welding and structural integrity retesting procedures to confirm the pressure load characteristic prior to placing the turbine back into operation. These connections are very expensive to install and future washes would still require time to remove the cap for cleaning and the subsequent reinstallation of the cap following completion of cleaning. Future use of this cleaning method including preparing for wash and restoring the turbine afterwards would cost at least $6000 every time used. Lost generation in to a 24 hour period would cost the power company at least $174,960 in replacement power costs.